Remote temperature measurement has generally been limited to electrical techniques. Thermocouples, thermistors, RTD's and semiconductor devices have generally been used as the temperature responsive elements, producing temperature responsive signals in accordance with either junction voltage changes or bulk resistance changes. The temperature-sensitivities of such devices are low, (0.04 mV per degree C. or 0.4% per degree C. being typical,) and the signals are often masked by noise from electrical equipment in the vicinity.
A particularly serious disadvantage of the conventional electrical methods, in certain applications, is that the electrical conductors between the sensors and remote controllers present dangers of explosion or fire in the event of electrical breakdown or short circuit.
In order to avoid the disadvantages mentioned above, numerous fiber optical devices have been proposed for temperature measurement in recent years. One such device is disclosed in U.S. Pat. No. 4,136,566, issued Jan. 30, 1979, to Christensen. In this device the temperature sensing element is a semiconductor such as gallium arsenide which absorbs monochromatic radiant energy as a function of temperature. Radiant energy from a monochromatic source is transmitted via optical fibers to the element and returned via optical fibers to a photodetector whose output corresponds to the temperature to be measured. A problem with devices of this type is that their accuracy is limited by thermally induced spectral shifts in the monochromatic source. Another problem, when such devices are used for remote temperature measurement, is that they must be calibrated in situ since transmission losses would otherwise affect the calibration.
Another fiber optical measuring apparatus which overcomes the latter problem is described in U.S. Pat. No. 4,278,349, issued on July 14, 1981 to Sander. In this case the radiant energy is transmitted to the sensing element at two different wavelengths, the absorption at one of the wavelengths being temperature-dependent and the absorption at the other wavelength being unaffected by temperature changes. The temperature to be measured is determined by comparing the detector responses at the two wavelengths. However, the accuracy of this apparatus is also affected by thermally induced drift.